1. Field of the Invention
The present invention relates to a particle-optical component which images or deflects a beam of charged particles, and the invention further relates to a system which comprises such a particle-optical component. The invention relates in particular to a magnetic lens assembly, the optical axis of which can be shifted relative to a geometric symmetry axis of the assembly. Moreover, the invention relates in particular to a projection system for transferring a pattern defined on a mask onto a particle-sensitive substrate by means of a beam of charged particles, in particular to an electron beam projection lithographic system. Furthermore, the invention relates in particular also to an electron microscopy system.
2. Background
Magnetic lens configurations (or assemblies), which act on charged particles of a given energy like a lens having a focal length f, are known in the prior art. With such a magnetic lens, it is possible to focus a parallel particle beam in an image plane positioned away from the lens by a distance f. The imaging quality of the bundle of beams passing centrally through a symmetry axis of the magnetic lens is comparatively good. However, if a bundle of beams to be imaged passes decentrally through the magnetic lens, an imaging in the image plane results in aberrations that are too high for practical applications.
The article xe2x80x98MOL (Moving Objective Lens)xe2x80x99, Optik 48 (1977), pages 255 et seq., by E. Goto et al., proposes shifting the optical axis of the lens from the symmetry axis of the lens by means of a corrective magnetic field such that it coincides with the decentral particle beam.
U.S. Pat. No. 4,376,249 discloses a concrete realization of such a lens as proposed by E. Goto and is referred to therein as xe2x80x98variable axis lensxe2x80x99. The magnetic lens disclosed therein comprises two pole ends which are axially spaced apart from one another relative to a geometric symmetry axis of the lens and are facing radially inwardly to the symmetry axis. Positioned radially inside these pole ends is respectively provided a magnet assembly for producing a magnetic field such that the optical axis can be shifted away from the symmetry axis. The lengths and diameters of the appertaining coils are so dimensioned that they produce a magnetic field transverse to the symmetry axis whose magnitude is proportional to the first derivative of the axial component of the magnetic field of the lens along the symmetry axis.
It has been found that for certain applications the conventional assembly known from U.S. Pat. No. 4,376,249 does not provide the necessary precision.
European patent 0 969 326 A2, the contents of which is fully incorporated herein by reference, discloses a lithographic apparatus for transferring a pattern defined on a mask to a radiation-sensitive substrate by means of an electron projection system. To this end, the mask is scanned in stripes by means of an electron beam having a well-defined beam cross-section, wherein a portion of the beam which passes unscatterd through the mask is likewise imaged in stripes onto the substrate. In order to ensure sufficient exposure quality, the beam cross-section must be defined with sufficient accuracy in the mask plane. If this is not the case, the substrate is either underexposed or overexposed at certain points.
It is desirable to have an apparatus capable of deflecting a beam of charged particles with less abberations.
One aspect of the invention relates to a deflection arrangement (or assembly) for a beam of charged particles which is provided such that a beam cross-section is transferred from an object plane of the arrangement to an image plane, wherein after projection the beam direction the beam cross-sections in the object plane and the image plane are shiftable relative to one another transverse to the beam direction.
The deflection arrangement comprises a magnetic lens doublet having two magnetic lenses. The two magnetic lenses are positioned relative to one another such that an object positioned in a front focal plane of the first lens, also referred to as collimator lens, is imaged to infinity. The second magnetic lens, also referred to as projection lens, is positioned such that its front focal plane coincides with the rear focal plane of the first magnetic lens. The second magnetic lens thus focuses the beam collimated by the first magnetic lens to the rear focal plane thereof which is thus the image plane for the doublet.
In one embodiment, a beam shifting arrangement is positioned between the first magnetic lens and the second magnetic lens providing such a magnetic field that a beam entering the beam shifting arrangement emerges from the beam shifting arrangement shifted parallel to its original beam direction.
Other aspects of the invention relate to electron microscopy systems employing a deflection arrangement for a beam of charged particles according to the embodiment of the invention, e.g., a scanning electron microscope (SEM) and a transmission electron microscope (TEM). In the scanning electron microscope electrons emitted from an electron source are focused onto a sample mounted on a sample holder by means of a focusing assembly, and electrons emerging from the same due to an interaction of the electrons focused onto the sample are detected by a detector. The deflection arrangement then forms part of the focusing assembly and enables the electron beam to be precisely focused onto the sample, with a large area scanned by the electron beam, i.e., a large observable image field, being provided.
The transmission electron microscope comprises an electron source for a large area illumination of a sample mounted on a sample holder as well as a position sensitive detector onto which transmission electrons emerging from the sample are imaged by a lens arrangement. In this case, the deflection arrangement forms part of the lens arrangement and enables the electrons emerging from the sample of an arbitrary partial field of a large area to be imaged with high imaging quality onto the position sensitive detector.
Another aspect of the invention relates to a magnetic lens arrangement which is referred to in the prior art as xe2x80x98moving objective lensxe2x80x99 or xe2x80x98variable axis lensxe2x80x99. Such a magnetic lens arrangement comprises in particular a focusing lens device for providing a magnetic field which is substantially rotationally symmetrical in respect of a symmetry axis of the arrangement, said magnetic field acting on the beam passing through said magnetic field as a focusing lens with an optical axis, said magnetic lens arrangement comprising moreover preferably an axis shifting device for producing a corrective magnetic field which is superposable on the magnetic field provided by the focusing lens device and acts on the beam such that the optical axis is shiftable parallel to the symmetry axis of the arrangement. As a result, the same focusing action is imparted on a particle beam entering the lens shifted from the (geometric) symmetry axis of the lens as on a particle beam entering the lens centrally, so that aberrations resulting from the decentral entry into the lens device being largely suppressed.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.